Electrochemistry of Controlled-Diameter Carbon-Nanotube Fibers at the Cross Section and Sidewall

Electrochemistry at open ends and sidewalls of carbon nanotubes (CNTs) has been under debate, with opposing viewpoints as to which sites are more electro-chemically active. A particular challenge in this field has been the ability to conduct electrochemical studies at the open ends of CNTs, without measuring contributions from the sidewalls. Among the detailed studies on CNT electrochemistry by various research groups, no reports show a direct comparison between the electrochemistry of open ends and sidewalls. That is to say, electrochemical studies at open ends have always had some contribution from sidewalls. Multiple research groups have reported electrochemistry at CNT sidewalls and close ends, although there are no reports that examine the electron transfer rate at open ends exclusively. In cases where rapid electron transfer at open ends has been reported, this has been on either defective dispersed CNTs at an electroactive surface, or on vertically aligned CNTs which are inherently porous. More interestingly, it was also observed that the electrochemical response at CNT modified electrode surfaces could be dominated by thin layer cell behavior, which arises due to the change in diffusional regime or mass transport. Therefore, in such electrode assemblies, we cannot completely assign the electrochemical response to open ends since sidewalls also contributed significantly to the measured current. In this work, we report on electrochemical measurements taken from the sidewall and the cross-section ( i.e., open ends) of highly densified multi-walled carbon nanotube fiber (HD-CNT f s) embedded in a polymer matrix. The cross section of the HD-CNT f s was exposed to reveal only open ends to the electrolyte interface and this platform allowed us to compare electrochemistry at the open ends and sidewalls of HD-CNT f s. Cyclic voltammetry was employed to examine the electrochemical properties of HD-CNT f s using a conventional bulk electrochemical cell and micro-capillary electrochemical measurements. The cross sections and side walls of HD-CNT f s are shown to promote similar electron transfer kinetics with reversible redox behavior. However, side walls of HD-CNT f s also showed large capacitive background current which was found to increase in proportional to CNT fiber diameter. Figure 1